Conversion coatings are employed on metals, notably aluminum and aluminum alloys whereby the metal surface reacts with a solution to convert to a corrosion protective film. Often, but not always, this protective film serves as a primer which may be top-coated with a paint for appearance purposes and also to enhance corrosion resistance. Heretofore, conversion coatings have employed chromates where maximum corrosion protection is desired or required. The most widely used chromate treatment for aluminum is the chromate-containing Alodine 1200 process (Alodine 1200 is manufactured and sold by Amchem Products, Inc., Ambler, Pennsylvania). The Alodine process, however, puts chromates into waste water which are either not permitted or are severely restricted by the Environmental Protection Agency of the United States Government. Illustrative of such chromate uses in protective coatings are the U.S. Pat. Nos. 4,146,410 to Reinhold and 4,541,304 to Batiuk and the prior art references cited therein.
Where efforts have been made to avoid the use of chromates in conversion coatings special treatments are required which in most cases are either objectionable and unacceptable or do not provide the required or desired degree of corrosion resistance. Illustrative of such non-chromate coatings are the following U.S. Pat. Nos. 3,672,821 issued to Schlussler and 3,964,936 issued to Das. Also and more closely related to the present invention is the Great Britain patent 2 195 338A issued to Sanchem, Inc. and Paper No. 197 from CORROSION 86, entitled "Cationic Film Forming Inhibitors for the Protection of 7025 Aluminum Alloy Against Corrosion in Aqueous Chloride Solution" by Arnott, Hinton and Ryan presented at the annual meeting of the National Association of Corrosion Engineers, March 17-21, 1986.
The Sanchem patent proposes a non-toxic conversion coating process employing relatively high alkaline solutions (pH 7 to 14) and is limited to in-house or laboratory use because of the elevated temperatures (at least 150.degree. F.) required. Moreover, the coating produced by Sanchem has limited corrosion inhibition, not acceptable in severe aqueous saline environments, notably MIL-C-5541C referred to above.
The Arnott et al. article recognizes the use of cerous chloride in lieu of a chromate to improve corrosion inhibition of aluminum. However, to be effective, exposure of the aluminum specimens to the cerous chloride is required for a prolonged time, on the order of 65 hours, which is unacceptable in production use. Moreover, the coated aluminum still fails to meet the corrosion protection requirements in severe aqueous saline environments.
Separately and apart from the foregoing, present day conversion coatings as illustrated by the above cited patents and publication, are readily wetted by moisture. It is well known that corrosion resistance of coatings is not as good as it could be if moisture were repelled, i.e, the coating were hydrophobic. At the same time there is a problem in making surfaces hydrophobic. Paint topcoats will not adhere to surfaces which are highly hydrophobic, i.e., surfaces which have too low a surface energy. Surfaces readily wetted by water have energies greater than 65 dynes/cm. while surfaces such as polyethylene and teflon which have surface energies of approximately 25 dynes/cm. are not readily wetted by moisture or solvents. Consequently it is difficult to get adequate paint adherence on surfaces having low energy. However, it was demonstrated that the standard epoxy-polyamide paint (MIL-P-23377) used on Air Force and Navy aircraft will adhere well to surfaces having an energy at or above 40 dynes/cm. The results of this study are shown in the following Table.
TABLE I ______________________________________ CRITICAL SURFACE TENSION OF WET- TING OF CLEANED PANELS (dynes/cm) 7075-T6 7075-T6 7178-T6 Cleaning Bare Clad Bare PAINT Method* Aluminum Aluminum Aluminum ADHESION** ______________________________________ 1 55.4 63.5 56.7 Passed 2 59.5 68.8 58.0 Passed 3 29.4 27.5 13.0 Failed 4 13.0 36.2 13.0 Failed 5 32.0 36.2 36.2 Marginal 6 16.0 16.0 32.0 Failed 7 49.2 54.0 55.4 Passed 8 27.5 32.0 40.0 Passed 9 49.2 58.0 62.0 Passed ______________________________________ * Method 1 -- This method consisted of brushing a coat of Turco 4906 (a product manufactured and sold by Turco Products Division of Purex Corp., Wilmington, California) on the panels, rinsing with water, neutralizing with 5% by weight aqueous NaHCO.sub.3, and again rinsing with water. The cleaner remained on the panels for 15 minutes before the first rinse. Method 2 -- A layer of Chemidize 727C (a product manufactured and sold by Hughson Chemicals, Erie, Pennsylvania) 5 to 10 mils thick, was applied to the contaminated panels and rinsed with water after 15 minutes. Method 3 -- The panels were wetscrubbed with SCOTCHBRITE No. 447 Type A pads (a product manufactured and sold by 3M, Inc., Minneapolis, Minnesota wetted with methyl ethyl ketone with moderate pressure and just long enough to abrade the surface to brightness. The loose powder formed by th scrubbing operation was removed with paper towels wet with methyl ethyl ketone. Method 4 -- The panels were soaked for 15 minutes in a solution of Clarkson AQS Emulsion (a production manufactured and sold by Clarkson Chemical Company, Palo Alto, California) diluted to the manufacturer's specifications, and then rinsed with water. Method 5 -- The substrates were solventcleaned. Texize 882 (a product manufactured and sold by Tec Chemical Co., Monterey Park, California) was applied for 15 minutes; the surfaces were then rinsed with water and dried. Method 6 -- The panels were wiped with paper towels wet with methyl ethyl ketone solvent. They were then scrubbed to brightness with SCOTCHBRITE No 447 Type A pads wet with water, given a water rinse, and a final methyl ethyl ketone solvent wipe. Method 7 -- The substrates without surface treatments were solventcleaned (methyl ethyl ketone). Texize 882 emulsion cleaner was applied for 15 minutes, rinsed with water, dried, and then coated with Spray Coating 13 (a product manufactured by Spraylat Ltd., Mt. Vernon, New York) to protec the surfaces from contamination. Method 8 -- Texize 820 (a product manufactured and sold by Tec Chemical Co. Monterey Park, California) diluted according to the manufacturer's directions, was applied with a brush and permitted to remain on the panel for 15 minutes. It was then rinsed off with water at room temperature. Method 9 -- The panels were cleaned by applying a layer of Turco 4906, 5 to 10 mils thick, and rinsing with water. They were then treated with a solution containing 5% Na.sub.3 PO.sub.4 and given a final water rinse. ** Tests were conducted with SCRATCHMASTER (Tradename of a paint adhesion tester of Dupont Chemical Co., Wilmington, Delaware). The SCRATCHMASTER measures paint adhesion by moving a blade over a painted surface with a gradually increasing load. The load, in kilograms, required to scrape through the paint to base metal is a quantitative measure of the paint adhesion.